Conventionally, methods of forming a circuit substrate having a circuit pattern on an insulative substrate can be performed by insert molding the circuit pattern into the insulative substrate or by laminating the circuit pattern with the insulative substrate. However, the aforesaid conventional methods can undesirably increase the thickness of the circuit substrate. Moreover, when the circuit pattern is modified or changed, adjustment of manufacturing equipments in the processing steps of the conventional method is time consuming.
U.S. Pat. No. 4,865,873 discloses a method for making a circuit substrate having a circuit pattern on a substrate. The method includes forming an insulating layer on a substrate, forming a water-soluble layer on the insulating layer, forming a patterned hole extending through the water-soluble layer and the insulating layer by laser ablation, forming an active metal layer in the patterned hole and on the water-soluble layer, and simultaneously electroless depositing a primary metal layer on the active metal layer and dissolving the water-soluble layer in an aqueous plating solution. Since the active metal layer covers a hole wall of the patterned hole as well as the water-soluble layer, electroless plating of the primary metal layer takes place not only at the hole wall but also at the surface of the water-soluble layer, which is undesirable. Although the water-soluble layer will be gradually dissolved in the aqueous plating solution during electroless plating, it can have an adverse effect on electroless plating. In addition, the thickness of the circuit substrate thus formed is considerably increased.